ON THE MEAN TEMPERATURE OF THE REGIONS IN WHICH THE GLACIÈRES OCCUR.
Many interesting experiments have for long been carried on with a view
to determine the mean temperature at various depths below the surface of
the earth. The construction of Artesian wells has afforded useful
opportunities for increasing the amount of our knowledge on this
subject; and the well at Pregny, near Geneva,[211] and the Monk Wearmouth
coal-mines, as observed by Professor Phillips while a fresh shaft was
being sunk,[212] have supplied most valuable facts. Without entering
into any detail, which would be an unnecessary trouble, it may be stated
generally, that, under ordinary circumstances, 1° F. of temperature is
gained for every 50 or 60 feet of vertical descent into the interior of
the earth. I have only met with one account of an experiment made in a
horizontal direction, and it is curious that the law of the increase of
temperature then observed seemed to be very much the same as that
determined by the mean of the vertical observations. Boussingault[213]
found several horizontal adits in a precipitous face of porphyritic
syenite among the mountains of Marmato. In one of these adits--a gallery
called Cruzada, at an elevation of 1,460 mètres--he found an increase
of 1° C. of mean temperature for every 33 mètres of horizontal
penetration, or, approximately, 1° F. for 60 feet.[214]
Again, observations have been made, in various latitudes, of the
decrease of temperature consequent upon gradual rising from the general
surface of the earth; as, for instance, in the ascent of mountains.
Speaking without any very great precision, but with sufficient accuracy
for ordinary purposes, 1° F. is lost with every 300 feet of ascent.[215]
It is evident that this decrease will be less rapid where the slope of
ascent is gradual, from such considerations as the angle at which the
sun's rays strike the slope, and the larger amount of surface which is
in contact with a stratum of atmosphere of any given thickness.
With these data, it is easy to arrive at some idea of the probable mean
temperature of the rock containing several of the glacières I have
described. The elevation of some of them has not been determined with
sufficient accuracy to make the results of any calculation trustworthy;
but four cases may be taken where the elevation is known--namely, the
Glacières of S. Georges, S. Livres, Monthézy, and the Schafloch. If we
take as a starting point the mean temperature of the town of Geneva,
which has been determined at 49°·55 F., the elevation of that town being
nearly 1,200 feet, we obtain the following approximate results for the
mean temperature of the surface at the points in question:--
S. Georges .... 40°·22 Fahr.
S. Livres (Lower) .... 38°·55"
Schafloch .... 33°·88"
Monthézy .... 41°·55"
The law of decrease of temperature enunciated by M. Thury gives a higher
mean temperature for the surface of the earth in these places, as in the
following table:--
S. Georges .... 41°·8 Fahr.
S. Livres .... 40°·1"
Schafloch .... 35°·6"
Monthézy .... 42°·5"
If any certain information could be obtained of the elevation of the
Abbey of Grâce-Dieu, I am sure that a result more surprising than that
in the case of the Glacière of Monthézy would appear. The elevation of
the floor of the church in the citadel of Besançon is 367·7 mètres, and
the plateau on the north side of the town of Baume-les-Dames is 531·9
mètres. I am inclined to think, from the look of the country, that the
latter possesses much the same elevation as the valley in which the
Abbey lies; and in that case we should have comparatively a very high
mean temperature for the surface in the neighbourhood where the glacière
occurs.
But if these are the mean temperatures of the surface, the natural
temperatures of the caves themselves should be still higher, on account
of the allowance to be made for increase of temperature with descent
into the interior of the earth. This element will very materially affect
our calculations in such a case as the lower part of the ice in the
Glacière of the Pré de S. Livres, and the strange suggestive beginning
of a new ice-cave 190 feet below the surface, on the Montagne de l'Eau,
near Annecy. In any open pit or cave, the ordinary atmospheric
influences find such easy access, that the temperature cannot be
expected to follow the law observed when perforations of small bore are
made in the earth, as in the case of the preliminary boring before
commencing to dig a well;[216] but the two glacières mentioned above are
so completely protected in their lowest parts, that they may be treated
as if they were isolated from external influence of all ordinary kinds;
and it may fairly be said that the mean temperature there ought to be
considerably higher than at the surface.
It is not very likely that the results of the above calculations are
strictly in accordance with what a careful series of observations on the
spot might show. The distance between Geneva and the Glacières of S.
Georges and S. Livres is sufficiently small to make it probable that the
reality is not very far different from the calculated temperature; but
the other two caves are comparatively so far off, that the temperature
and elevation of Geneva are not very safe data to build upon.
FOOTNOTES:
[Footnote 211: Bischof, _Physical Researches_, 189.]
[Footnote 212: _Philosophical Magazine_, v. 446 (1834).]
[Footnote 213: _Annules de Chimie et de Physique_, liii. 2-10. See also
Bischof, 136.]
[Footnote 214: The English edition of Bischof affords here a proof of
the danger of frequent changes from one scale to another. Bischof in the
first instance rendered Boussingault into degrees Réaumur, and this was
in turn reduced to degrees Fahrenheit; the result being that the
authorised English edition of his book gives 2°·25 F. for 127·5 feet,
which does not come within 10 feet of Boussingault's statement.]
[Footnote 215: M. Thury calculates a decrease of 1° C. for every 174
mètres between Geneva and S. Bernard, which is less than the decrease
given in the text. He arrives at this conclusion by correcting the mean
temperature of Geneva from 8°·9 C., the observed mean of eighteen years,
to 9°·9 C., in consequence of supposed local causes, which unduly
depress the temperature of Geneva. With the mean 8°·9 C. a result nearly
in accordance with that of the text is obtained.]
[Footnote 216: Professor Phillips found, in the course of his
investigations in the Monk Wearmouth mines, some hundreds of yards below
the sea, that when a new face of rock was exposed, its temperature was
considerably higher than that of the gallery or shaft in which it lay.
In some cases the difference amounted to 9 and 10 degrees. The rock soon
cooled down to an agreement with the surrounding temperature.]
* * * * *
APPENDIX.
M. Thury's observations during his winter visit to the Glacière of S.
Georges are so curious and valuable, that I give the principal results
of them here.
It will be remembered that this glacière consists of a roomy cave, 110
feet long and 60 feet high, with two orifices in the higher part of the
roof, one of which is kept covered with the trunks of trees to shut out
the direct radiation of the sun. A little thought suggested to M. Thury
that the cold in the cave in mid-winter would most probably be greater
than the external cold of the day, and less than that of the night; so
that there should be a time in the later evening when a column of colder
and heavier air would begin, to descend through the hole in the roof. To
test the correctness of this supposition, he took up his abode in the
cavern for the evening of the 10th January, 1858, with a lighted candle.
The flame burned steadily for some time; but at 7.16 P.M. it began to
flicker, and soon inclined downwards through an angle of about 45°; and
when M. Thury placed himself under the principal opening, the flame was
forced into an almost horizontal position. At 8 P.M. the current of air
had all but disappeared. This violent and temporary disturbance of
equilibrium was a matter of much surprise to M. Thury; for he had
naturally expected a quiet current downwards, continuing through the
greater part of the night.
At 7.16 P.M. the external temperature was 23·9° F., and the temperature
of the atmosphere in the cave at the same time was 30°·88 F.;[217] so
that there is no wonder the current of air should be strong. It is very
difficult to say, however, why it did not commence much earlier,
considering that the external air must have been heavier than that in
the cave long before 7 o'clock. M. Thury refers to the mirage as a
somewhat similar instance, that phenomenon being explained by the
supposition that atmospheric layers of different temperatures lie one
above another in clearly-defined strata. He suggests, also, that as the
heavier air tends to pass down into the cave, the less cold air already
in the cave tends to pass out; and the narrow entrance confining the
struggle between the opposing tendencies to a very small area, the
weaker initial current is able for a time to hold its own against the
intruder. On this supposition, it is easy to see that when the rupture
does occur it will be violent.
The next day, M. Thury arrived at the glacière at 9.50 A.M. He had
determined, in the summer, that the temperature of the cave was
invariable, at any rate through the 3-1/2 hours of his visit (from 7.30
to 11 A.M.); but his winter experience was very different. The following
are the results of his observations.
In the cave:--
January 9, at 7.16 P.M.[218]... 30°·884 Fahr.
" " 7.20 " ... 29°·75 "
" " 7.27 " ... 27°·5 "
" " 7.50 " ... 26°·834 "
January 10, at 10.12 A.M. ... 23°·684 "
" " 10.0 " ... 23°·9 "
" " 11.20 " ... 24°·022 "
" " 12.14 P.M. ... 24°·134 "
" " 1.30 " ... 24°·35 "
" " 2.30 " ... 24°·584 "
" " 3.14 " ... 24°·8 "
" " 4.0 " ... 25°·142 "
Supposing the weather to have been much the same on the 9th and 10th of
January, as M. Thury's account seems to say, there is something very
strange in the great difference between the temperatures registered at 4
P.M. on the one day, and at 7.16 P.M. on the other.
The external temperatures at the mouth of the cave were as follows:--
January 10, at 10.53 A.M. 25°·934 Fahr.
" " 11.14 " 26°·384 "
" " 11.45 " 28°·04 "
" " 12.32 P.M. 27°·944 "
" " 1.12 " 30°·644 "
" " 3.3 " 26°·834 "
" " 3.56 " 25°·7 "
" " 4.26 " 25°·25 "
The minimum temperature of the external air during the night of January
10-11 was 18°·392 F., and that of the glacière 19°·76 F.[219] During the
preceding night, the minimum in the cave was 22°·442 F., which may throw
some light upon the difference between the temperatures at 7.16 P.M. on
the 9th, and at 4 P.M. on the 10th.
M. Thury bored a hole, of about 10 inches in depth, in the flooring of
ice, and placed a thermometer in it, at 12.25 P.M., closing it up with
cotton. At 2.55 P.M., and at 4.7. P.M., the thermometer marked the same
temperature, namely, 26°·24 F.
M. Thury's views on glacières in general, based upon the details of the
three which he has visited, are much the same as those which I have
expressed. He has, however, more belief than I in 'cold currents.'
FOOTNOTES:
[Footnote 217: This was given by a thermometer only placed in the cave
at 7 P.M., and by construction not very sensible.]
[Footnote 218: The moment when the disturbance of the atmosphere
commenced.]
[Footnote 219: M. Thury gives--4°·62 C. as the minimum in the glacière
during the night in question; but on the next page he gives--6°·8 C.
(=19°·76 F.). It is evident, from a comparison with other details of his
observations, that the latter is the correct account.]
* * * * *